# Water Analogy and Incompressibility

Discussion in 'Physics' started by mOOse, Aug 28, 2007.

1. ### mOOse Thread Starter Member

Aug 22, 2007
20
0
In the water analogy, the pressure is higher before a resistor (restriction)
than after it, just like a voltage drop. But does that mean that the density of
water is higher before the restriction than after it? It seems logical, but
I've heard of the "incompressibility" of water and am wondering what that
means (and if its just an ideal). In the water analogies, they sometimes talk
of "mass flow rate" and sometimes of "volume flow rate". I guess if water is
"incompressible" in the sense that it's mass density does not change under
pressure changes, then these two kinds of flow rate would be equivalent.

http://en.wikipedia.org/wiki/Hydraulic_analogy
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
If fluids were compressable, then modern hydraulic equipment (including automobile brakes) would not work. Water's mass density is greatest at 4 deg. C.

I don't find that current in a wire and water flowing through a pipe relate very well.

3. ### niftydog Active Member

Jun 13, 2007
95
0
I found the water analogy useful in the first few weeks of my diploma - but if you analyse it too much you soon discover many inconsistencies that can give you the wrong impression. I would urge you to consider what's actually happening rather than trying to reconcile reality with the analogy.

4. ### recca02 Senior Member

Apr 2, 2007
1,211
0
water(most real fluids) is compressible to a very small extent meaning they require huge force to make any noteworthy changes in volume.
hence density changes by a small value.
from what i think mass flow rate is good for consideration where water may change to steam like in high pressure boilers where their density changes with both temp and pressure. also since most calculation of enthalpy and work done involve mass.
in thermodynamics we use specific volume which is inverse of density.

5. ### steeve_wai Active Member

Sep 13, 2007
47
0
DEAR FRIEND,
ANALOGIES SELDOM GIVE THE CORRECT PICTURE.i have experienced the same myself.
anyway here is a link that provides yet another analogy.do ponder over the question given below, after you have visited the link.

http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/DC-Current/WaterFlowAnalog.html

NOW TELL ME WHAT WILL THE DIAGRAM LOOK LIKE IN THE CASE OF A ZERO RESISTANCE WIRE, A SUPERCONDUCTOR FOR INSTANCE.
good luck

6. ### ixisuprflyixi Active Member

Sep 16, 2007
52
1
Moose, I could be wrong but when you were told about "mass flow rate" and "volume flow rate" I think the analogy was just trying to convey the idea of voltage and current flow. Current being the mass or amount of flow and volume of charge being electromotive force or voltage. Like the other guys say just use the water analogy to get a hang of what voltage and current may be. Once you get past, voltage is like water pressure and current is like amount of water flow you can pretty much let the water analogy go. Water compressability and thinking of fluid mechanics and dynamics will really start to screw with you . hope that helps